A CMOS Power Management (PM) Integrated Circuit (IC) is used in a number of applications, including, for example, battery power supplies for handheld electronic devices such as WLAN portable devices, multimedia devices, and portable music players. Battery power for operating these handheld electronic devices is often supplied by a low voltage battery, such as, for example, a two-cell Alkaline battery that provides a terminal voltage over a voltage range of 1.8 to 3 volts. Battery power for these handheld electronic devices is also sometimes supplied by a battery with a higher voltage, such as a Lithium Ion battery that provides a terminal voltage over a voltage range of 3 to 4.2 volts.
A PM IC can include an auxiliary circuit block, such as a Real Time Clock (RTC) circuit that provides time and data information. An auxiliary circuit block of this type often needs to be operational even when the main batteries are not available in order to maintain, for example, time and data information in a RTC circuit. In this case, a back-up battery supplies the RTC circuit when the main battery cannot supply power to the PM. When the main supply battery of a voltage regulator for a low-current real time clock (RTC) circuit is removed, a 3 volt Lithium-Ion back-up battery ensures that the RTC circuit can operate for a relatively long time without a main supply battery.
However, problems can arise if the main supply battery is provided, for example, by a two-cell alkaline battery that has a voltage range of 1.8 to 3 volts. In this case, the two-cell alkaline main battery could not provide sufficient voltage by itself to directly supply the RTC circuit and also to charge a back-up battery, such as a 3.0 volt Lithium-ion. In this case, a power converter is needed that is able to generate a voltage that is higher than the voltage provided by the two-cell alkaline main battery and that charges the back-up battery.
Power management for a power converter that accommodates several different types of main supply batteries is not very efficient for use in conjunction with a low-current type of auxiliary RTC circuit in a PM IC. In a typical CMOS process with, for example, a 0.35 micron technology, a RTC block can be supplied by a 3 volt battery with a lower supply current that is in the range of 1 to 10 microamps. Using a higher voltage battery, such as Li-Ion battery, a low-current RTC circuit can be powered using only a low-power, LDO voltage regulator. On the other hand, using an Alkaline battery requires that the RTC circuit must be supplied with a higher voltage from a voltage step-up power converter that steps-up the Alkaline battery input voltage to a higher output voltage level. From a power efficiency point of view, a step-up power converter is inefficient. In fact, a step-up power converter, such as, for example, a regulated charge pump, uses digital and analog circuits that require currents of at least some tens of microamps. Consequently, there is a need for a power converter that has very low power consumption and that can also efficiently supply power from both low voltage Alkaline batteries and from higher voltage Lithium-ion batteries.
FIG. 1 is an example of a conventional regulator and charger configuration 10 for a low-current RTC circuit and a back-up battery. A main supply battery 14 provides at an output terminal 16 an output voltage vbat to a power converter 18. The power converter 18 provides a regulated output voltage vout at the output terminal 20 to power the RTC circuit 12. The output terminal 20 of the power converter 18 is connected through a series resistor 22 (used to limit the back-up battery charge current) to a back-up battery terminal 24 at which is provided a voltage vbck for charging a back-up battery 26. The back-up battery 26 is represented as a capacitor Cbk that is connected between the back-up battery terminal 24 and a ground terminal 28. A typical back-up battery provides a voltage of 3 volts.
Prior art for a regulated power supply based on a charge pump is disclosed in U.S. Pat. Nos. 5,694,308; 6,300,820; 6,456,153; and 6,873,203. These charge pump patents have two common characteristics: 1) for good power efficiency, the regulated output voltages must be greater than the main power supply voltage; and 2) the circuits shown in these patents are not optimized to efficiently supply a low-power block, such as RTC circuit.